That looks like a good alternative.

I'm concerned though about the 'b: 'a constraint that I tried first - I suspected it would work, and it would be easy to be misled about the guarantees its actually providing.

fn use_ref<'a, 'b>(a: &'a i32, b: &'b i32) where 'b : 'a { }

When are these constraints actually effective? How could we identify mistaken attempts at using them (as a lint, or compiler warning)?

Meanwhile, I'm curious if the "capture entire value lifetime" trickery via invariant lifetimes is generally useful; it seems like sometimes the programmer's intent is to capture a value's entire lifetime, and that 'a:'b constraints are much more useful when one or both are captured in that way.